This invention relates to power-on reset circuits for holding electronic circuitry in reset when the supply voltage applied to the electronic circuitry is below a threshold voltage level and, in particular, to power-on reset circuits whose threshold voltage level is a function of the worst case device/component on the electronic circuitry.
Some form of power supply is required for every electronic circuit. Moreover, each electronic circuit requires some predetermined operating voltage range in order to operate properly. For example, an electronic circuit that requires a power supply of 5 volts may allow a voltage operating range of 4.5 volts to 5.5 volts for proper operation. However, if the power supply voltage falls below the 4.5 volts, the electronic circuit is not guaranteed to function properly, and if the power supply voltage rises above 5.5 volts, the electronic circuit may suffer irreversible damage.
Additionally, in complex electronic circuits such as microprocessors, microcontrollers or other control systems, a reset signal typically exists for holding the electronic circuit in a reset state until the power supply voltage exceeds a predetermined threshold voltage. Additionally, if the power supply voltage falls below the threshold voltage, the electronic circuit should be placed back into the reset state. Failure to reset the electronic circuit when the power supply voltage is below the threshold voltage may result in the electronic circuit functioning inadequately thereby causing catastrophic results. For example, where a microcontroller is used in an anti-lock brake system, inadequate operation of the microcontroller due to a low power supply voltage could cause failure of the braking system. Accordingly, it is of utmost importance to take the electronic circuit out of reset and operate only when the power supply voltage exceeds the threshold voltage. Moreover, once taken out of reset, the electronic circuit must again be placed back into the reset state if the power supply voltage later falls below the threshold voltage.
One scheme for resetting an electronic circuit upon start-up or power-on is to use external elements, such as resistors, capacitors and bipolar devices, to detect when the power supply voltage exceeds a predetermined threshold voltage. This scheme has the disadvantage that the predetermined threshold voltage is typically chosen to be an artificially high voltage level to insure that when the electronic circuit is taken out of reset, all of the various sub-circuits and devices in the electronic circuit are well within their proper operating voltage range. However, by requiring such an artificially high voltage threshold, the overall operating voltage range of the electronic circuit is substantially decreased.
Another scheme for holding an electronic circuit in a reset state upon power-on is to use a timer circuit, coupled for receiving the power supply voltage, including a capacitor that charges through a large resistor such that when the capacitor is fully charged, the electronic circuit is taken out of reset. However, this scheme provides only for a fixed time to hold the electronic circuit in reset while the voltage on power supply is ramping up and, thus, does not reliably handle slow power supply ramps. Accordingly, such schemes require a certain rise time specification on the power supply.
Another more advanced scheme for holding an electronic circuit in a reset state upon power-on is to use a circuit that generates power-on reset signals when the power supply exceeds a fixed DC trip point threshold. Such circuits typically include a comparator having one of its inputs coupled to receive a band gap voltage and the other input coupled to receive a scaled version of the power supply voltage, via a resistor divider circuit, such that when the scaled version of the power supply voltage exceeds the band gap voltage, the comparator trips and allows the electronic circuit to be taken out of reset. However, such an implementation requires substantial additional circuitry and the DC trip point threshold is generally chosen to be an artificially high voltage. Moreover, the DC trip point threshold is difficult to pinpoint due to normal manufacturing variances of the circuit components such as the offset errors in the comparator.
It is therefor a principal object of the present invention to provide an improved power-on reset circuit for generating a trip point threshold voltage that is based upon and tracks the worst case device/component (i.e., the component that requires the highest supply voltage to operate properly) on the electronic circuit thereby guaranteeing proper operation of the electronic device when taken out of reset while maximizing the operating voltage range of the electronic circuit.
It is a further object of the present invention to provide a power-on reset circuit that places the electronic circuitry back into a reset state if the power supply voltage later falls below the threshold voltage.